Epoxy resins are a class of thermosetting polymers widely used in structural adhesives, composites, surface coatings, and laminates due to their high strength, low creep, low cure shrinkage, resistance to corrosion, and excellent adhesion. A major shortcoming is the inherent brittle nature of the epoxies in the cured state, which inhibits various industrial applications. A well-known method employed to toughen brittle polymers is to incorporate a discrete phase of rubber particles into a rigid polymer matrix. A well-established factor responsible for the final mechanical properties of these hybrid materials, aside from the intrinsic properties of the composite materials themselves, is the resultant morphology of the material. Control over this morphology is critical for the fabrication of modified toughened epoxy composites with target end-use properties. One effective technique is to add a component capable of phase separation such as, a reactive liquid rubber or amphiphilic block copolymer. Traditional core-shell additives have also been used with limited success due to problems with dispersion efficiency. Another drawback of the core-shell additives is that they render the thermoset opaque (loss of transparency).
One well-known approach to toughened epoxy resins is through blending with a reactive “liquid rubber”. One commonly utilized material is a carboxyl-terminated copolymer of butadiene and acrylonitrile (CTBN). One benefit of these polymers is the ease of blending and minimal effect on viscosity due to the miscibility with the epoxy resin and the low glass transition temperature (Tg) nature of the material. The reactive CTBN polymers, when cured with an epoxy resin, increase in molecular weight, which in combination with the increasing resin molecular weight leads to a decrease in solubility of the rubber in the resin. The decreased solubility results in rubber phase separation. This separation results in discrete rubber particles covalently bound to the epoxy matrix. Unfortunately, incomplete phase separation leads to an undesirable reduction in the matrix Tg. Furthermore, the resultant phase separated morphology and thus the subsequent composite properties are not well controlled and are highly dependant on the cure system and cure profile. Lastly, CTBN contains a high level of unsaturation, which may lead to undesirable degradation and cross-linking reactions (especially at elevated temperatures) and they potentially contain trace carcinogenic free acrylonitrile. These deficiencies may be avoided with the acrylate system of the present invention.
A second approach to toughened epoxy resins is through amphiphilic block copolymers (epoxy miscible segment and epoxy immiscible segment) containing a rubbery component. These block copolymers are thought to toughen epoxy resins through the self-assembly of the amphiphilic block, which occurs prior to epoxy cure.
US2004/0034124 describes the use of amphiphilic block copolymers to toughen epoxy thermosets. Specifically claimed is the use of amphiphilic block copolymers containing PMMA. These block copolymers are shown to pre-assemble prior to the thermoset cure, thus enable toughening without compromising the resultant thermoset Tg. This provides benefit over the aforementioned CTBN type systems. The amphiphilic nature, coupled with the high Tg of the PMMA block imparts a significant viscosity increase in the pre-cured blend, which can be detrimental for many applications. Furthermore process difficulties can be encountered, as the dissolution of the PMMA block copolymer in the thermoset resin must be carried out at elevated temperatures. Also, the PMMA block while providing compatibility with the epoxy resin, does not contribute to the rubber toughening properties, or the effective rubber toughening efficiency is not optimal.
WO 2006/052727 is analogous to the abovementioned patent, but in place of PMMA it describes polyethyleneoxide (PEO)-based amphiphilic block copolymers having at least one polyether structure, for use as epoxy resin additives. These amphiphilic polyether blocks, at a use level of 1-10 weight percent on the epoxy resin, form nano-scale (15-25 nm) domains due to self-assembly (form into micellar structures in the cured epoxy system). One problem with PEO systems is that the PEO, while having a low Tg, is still crystalline (crystalline PEO homopolymers have a melting temperature (Tm)≈60° C.). The crystalline nature can affect the block copolymer ordering, reduce the effective rubber toughening efficiency, and process difficulties can be encountered, as the dissolution of the PEO block copolymer in the thermoset resin may need to be carried out at elevated temperatures. Also, PEO is water-soluble and readily absorbs moisture and care must be taken to limit moisture in epoxy cure applications as water has detrimental effects on the epoxy matrix Tg (significant reduction).
Furthermore, PEO-based block copolymers are produced using living anionic polymerization techniques. Living anionic polymerization suffers from several drawbacks, such as, poor copolymerization between polar and non-polar comonomers and the inability to use monomers that can be easily deprotonated. Therefore functional monomers cannot be directly incorporated and the copolymerization of monomer mixtures can be problematic and/or non-viable. This reduces the ability to tailor properties such as solubility, reactivity, and Tg. Furthermore, this process can be expensive, difficult or impractical to carry out on an industrial scale as bulk or emulsion techniques cannot be used, extremely pure reagents are necessary (even trace amounts of protic material inhibits polymerization), and an inert atmosphere is requisite. Lastly, the significance of tailoring block composition or allowing for the formation of gradient compositions to control solubility and final morphology are not taught.
WO 2006/052727 further describes other useful analogous amphiphilic block copolymers such as reactive poly(epoxyisoprene)-b-polybutadiene. Aside from the aforementioned limitations above, these types of structures are identified as complicated to prepare, requiring multiple steps, and therefore economically unattractive limiting application use. Furthermore, they have disadvantages due to a high level of unsaturation, which may lead to undesirable degradation and cross-linking reactions (especially at elevated temperatures).
Similar self-assembling amphiphilic block copolymers of poly(ethylene oxide)-b-poly(propylene oxide) for use in epoxy resin modification are also known (Macromolecules, 2000, 33, 5235-5244.)
US 2004/0247881 describes the use of an amphiphilic block copolymer as an epoxy modifier for a specific class of flame retardant epoxy resin. Examples are given of polyether-based block copolymers and of reactive poly(methyl methacrylate-co-glycidyl methacrylate)-b-poly(2-ethylhexylmethacrylate). Disadvantages of these types (High Tg and PEO-based) of block copolymers have been disclosed in the abovementioned text. US 2004/0247881 also mentions the possible use of low Tg methacrylic based block copolymers, but does not disclose the use or any benefits thereof.
Aside from the aforementioned limitations, none of these disclosures teach the significance of acrylic block copolymers containing all low Tg segments or the benefit of tailoring the two blocks through copolymerization or gradient block structures to control solubility and final morphology, which thereby control the resultant thermoset end-use properties.
Surprisingly, it has now been found that all low Tg (“liquid rubber” type) acrylic-based block polymers, either functionalized or unfunctionalized can be used to effectively modify thermoset resins and furthermore can be easily prepared and tailored to provide the desired modification. The acrylic-based block copolymers contain low Tg segments, rendering them easy to blend and use in current manufacturing processes, and providing toughness and/or flexibility to the thermoset resins. The acrylic block polymer is especially useful in modifying epoxy resins. The acrylic block polymers of the invention incorporate the advantages of “liquid rubbers” and block copolymers into the same material, providing a “liquid-rubber” type acrylic block copolymer. Furthermore, the process to make these materials is simple and cost effective and allows one the ability to readily tailor the polymer properties, such as Tg and solubility, through copolymerization and gradient structures. This tailoring allows one to control the final properties of the thermoset material.